The following may not be sent as merchandise at the First-Class Mail International price if they are liable to customs duty: Works of art (including photographs), printed forms, account books, manuscript books, labels, advertising matter (except trade catalogs and circulars), picture books, almanacs, maps, old paper, and old newspapers serving as packing paper.

Flat Rate Boxes — Medium and Large: The maximum weight is 20 pounds, or the limit set by the individual country, whichever is less. Refer to Notice 123, Price List, for the retail, commercial base, or commercial plus price.